The present invention relates to a system for controlling the shift of an automatic transmission and, more particularly, to a shift control system for controlling an oil pressure to be fed to frictional engagement means directly by a pressure regulator valve at a shifting time.
As is well known in the art, an automatic transmission for vehicles is constructed to change power transmission paths in a gear train to execute a shift by applying/releasing frictional engagement means including clutches and brakes. Since the shifting operation is followed by rotational fluctuations of the rotary elements including the engine, the output shaft torque is abruptly changed to cause a shifting shock thereby to deteriorate the riding comfortableness, if the frictional engagement means is abruptly applied or released. In the case of the so-called "clutch-to-clutch shift", in which a predetermined frictional engagement means is released whereas another is applied, the engine will be either blown up or tied up to lower the output shaft torque and the durability of the frictional engagement means if the applying or releasing timing is improper.
In the prior art, the engaging pressure of the frictional engagement means for absorbing the inertial energy accompanying the rotational fluctuations of the rotary elements is gradually augmented according to the characteristics of an accumulator by attaching this accumulator to the frictional engagement means.
The oil pressure for applying the frictional engagement means equipped with the accumulator will change, as illustrated in FIG. 9. For time period 1 after the feed of oil pressure is started, the piston of a hydraulic servo mechanism operates and advances to reduce the pack clearance. For subsequent time period 2, the oil pressure rises according to the characteristics of the accumulator so that the torque capacity of the frictional engagement means gradually increases.
If such accumulator is used, the frictional engagement means comes into gradual engagement, as described above, so that the inertial energy can be absorbed by the slipping actions of the frictional elements to effect a smooth shift. However, the accumulator having such action is required to have a considerable capacity of oil. For reducing the size and weight of the hydraulic control system, it is desired to control the engaging pressure of the frictional engagement means directly by a pressure regulator valve in place of the accumulator. This desirable control system is schematically shown in FIG. 10. According to this system, a pressure regulator valve CV for regulating the engaging pressure of frictional engagement means FD including clutches and brakes sets a regulated pressure level in accordance with the elastic force of a spring SP and a signal pressure coming from a linear solenoid valve or duty solenoid valve SOL and regulates a line pressure PL according to the regulated pressure level. The line pressure PL thus regulated is fed to the frictional engagement means FD.
In the construction shown in FIG. 10, the regulated pressure level is so controlled according to the signal pressure of the solenoid valve SOL that the oil pressure to be fed to the frictional engagement means may gradually rise via the aforementioned piston operation zone. As a matter of fact, the parts of the frictional engagement means have production dispersions in the stroke of its piston or in the load of its return spring. As a result, the control of the oil pressure in the piston operation zone may become improper to cause a surge (or overshoot) in the engaging pressure so that the shifting shock may grow more and more serious.
This will be specifically described in the following. The surge occurs when the piston reaches its stroke end, if the piston has such a dispersion that it will instantly complete its operation, as experienced in case the regulated pressure level of the pressure regulator valve deviates to a higher value, in case the piston stroke becomes shorter than a predetermined value, or in case the load of the return spring deviates to a lower level. Specifically, the effective area between a feed port of the line pressure in the pressure regulator valve and an output port to the frictional engagement means takes a large value when the engaging pressure is lower (allowing the oil to flow continuously) than the line pressure. If, in this state, the piston reaches its stroke end to interrupt the oil feed abruptly, a high flow of oil is abruptly interrupted because the effective area of the pressure regulator valve is not quickly reduced. As a result, the surge occurs in the engaging pressure. This behavior is illustrated in FIG. 11, in case the regulated pressure level is higher than that of the normal state. In FIG. 11, solid lines indicate the normal state. In case the regulated pressure level is higher, as indicated by broken lines, the surge occurs while the control belongs to the piston operation zone.
On the contrary, in the case of production dispersions in which the engaging pressure is gradually increased (or swept up) on or before the end of the piston operation, there arises the following phenomenon. Here, the production dispersions are experienced in case the regulated pressure level is lower than the predetermined value, in case the piston stroke is longer than the predetermined value, or in case the load of the return spring is higher than the predetermined value. The sweep-up of the engaging pressure is a control for raising the signal pressure of the solenoid valve SOL gradually to increase the engaging pressure accordingly gradually so as to establish the aforementioned accumulator characteristics. Unless the piston has finished its operation when the sweep-up is started, the oil continues flows into the chamber of the hydraulic servo mechanism so that the pressure regulator valve CV has its effective area increased, as in the foregoing cases. If, in this case, the piston reaches its stroke end, the flow of much oil is abruptly interrupted to cause the surge. This behavior is indicated by single-dotted lines in FIG. 11, in case the regulated pressure level is lower than the predetermined value.